1. Field of the Invention
The present invention relates to a method for manufacturing a silica glass base material for a wave guide preform such as an optical fiber preform, image fiber preform, light guide preform, rod lens preform, etc., used in the fields of optical communication and optical applications.
2. Description of the Related Art
A CVD method, such as the VAD (vapor phase axial deposition) method, the MCVD (modified chemical vapor phase deposition) method, the OVD (outside vapor phase deposition) method, the PCVD (plasma chemical vapor phase deposition) method, etc., the slip casting method, or a method combining the VAP method and the sol-gel method may be used to manufacture a wave guide preform such as an optical fiber preform, image fiber preform, light guide preform, rod lens preform, etc., which are used in the fields of optical fiber telecommunication and optical applications.
According to the CVD method, halide gas, such as SiCl.sub.4 or GeCl.sub.4, is used as a starting material, which is subjected to an oxidation reaction or flame hydrolysis reaction to produce oxide powder, such as SiO.sub.2 or GeO.sub.2. Further, the oxide powder is deposited on a specific deposition surface (distal end face of a target, inner peripheral surface of a silica pipe, outer peripheral surface of a mandrel, etc.), and is then consolidated into transparent glass, whereupon a preform is formed.
Also, manufacturing means for a preform based on the all-synthesis VAD method has recently started to be used in order to obtain optical fibers of higher quality.
In the manufacture of a preform for single-mode optical fibers, in this connection, porous glass bodies for a core and a part of cladding in a suitable outside diameter ratio are first simultaneously synthesized by the VAD method. Then, these porous glass bodies are consolidated into transparent silica-based glass rod. In order to adjust the core-to-cladding outside diameter ratio to 10/125 (.mu.m) (adjustment of outside diameter), thereafter, a porous glass body for cladding is deposited on the transparent silica-based glass rod by the OVD method, and the deposited body is consolidated into transparent glass preform.
According to the slip casting method, as is disclosed in Published Unexamined Japanese Patent Application No. 64-56331 and the like, first a slurry is formed by dispersing a preconditioned fine silica glass powder material in pure water, and is then poured into a molding die so that water in the slurry is absorbed by the die. Thus, a porous glass body is formed from the fine glass powder material. Thereafter, the porous glass body is dried and consolidated into transparent glass.
According to the method combining the VAD method and the sol-gel method, as is disclosed in Published Unexamined Japanese Patent Application No. 1-294548 and the like, a rod-shaped porous glass body (a combination of core glass and part of cladding glass), prepared mainly by the VAD method, and a tubular porous glass body (remainder of the cladding glass), prepared by the sol-gel method, are combined by using the rod-in-tube method. Thereafter, these porous glass bodies are consolidated together into transparent glass.
The CVD method, the slip casting method, and the method combining the VAD method and the sol-gel method, described above, have the following technical problems.
In the case of the CVD method, the efficiency of deposition of oxide powder is as low as 30 to 60%, although the technical achievement for conforming articles is high. The equipment size is very large. As mentioned above, there are some drawbacks to high yield, reasonable product cost, and economy of equipment.
The slip casting method permits high-efficiency production of high-quality porous glass bodies using simple equipment. However, a waveguide structure cannot be obtained with use of the technology of this method alone.
In the case of the slip casting method, therefore, the method itself must be improved, or otherwise, it requires use of some other suitable means. There are no technical suggestions, however, in the description of prior art examples related to the slip casting method.
In the case of the method combining the VAD method and the sol-gel method, the cost of the material, alkoxide, is high, and gilled matter is liable to crack when it is dried. Further, according to this method, it is difficult to obtain a large-sized base material.